At many formal rental places, most suit designs are only stocked in a size 40 regular; anything else must be ordered. As a result, anyone who isn't a size 40 regular will be unable to see how they would look in their preferred style of suit until such time as they actually rent it.

I would suggest
that it would be possible to design a suit which would look "reasonably normal" from the front for a range of sizes if the back of the suit, rather than being solid, had a bunch of adjustment straps. While one would obviously not go out in such a suit, it would allow the wearer to see how he would look in an appropriately-sized garment.

Every suit rental place I have ever been to or known anybody to visit has suits in a range of sizes, so you can try various different suits on, and pick one that you have tried and that you like; the idea of wearing a suit that you hadn't tried on at all, or even seen, doesn't sound too promising to me.

You wouldn't get an idea about how the suit hangs on you, meaning you might be able to tell if it fits but not how good it would look. Most clothes are not designed to be tied together at the back, and doing so would cause distortions and bulging.

On the other hand, this is slightly baked by tailors and dressmakers using pins to take in garments when they're tried on. Using pins at least gives more control as to how the fabric is adjusted.

pottedstu, on tv's "say yes to the dress" they
sometimes use big rubber tipped spring clamps to
take in the wedding gown samples being tried on.
of course it doesn't work if the brides are too big
for the dresses.
as far as this suit idea goes, i'm rather large, and
the tux places i've been to have always had
something to try on for size, but i do think this
idea has some merit.
or, you could just use the backless half suites
some undertakers use on bodies, if they still do
that.

oh wow! just noticed the dates on the other
posts, this thread is real old, and pre-dates "say
yes to the dress"!